[InstCombine] Fix for PR29124: reduce insertelements to shufflevector

If inserting more than one constant into a vector:

define <4 x float> @foo(<4 x float> %x) {
  %ins1 = insertelement <4 x float> %x, float 1.0, i32 1
  %ins2 = insertelement <4 x float> %ins1, float 2.0, i32 2
  ret <4 x float> %ins2
}

InstCombine could reduce that to a shufflevector:

define <4 x float> @goo(<4 x float> %x) {
 %shuf = shufflevector <4 x float> %x, <4 x float> <float undef, float 1.0, float 2.0, float undef>, <4 x i32><i32 0, i32 5, i32 6, i32 3>
 ret <4 x float> %shuf
}
Also, InstCombine tries to convert shuffle instruction to single insertelement, if one of the vectors is a constant vector and only a single element from this constant should be used in shuffle, i.e.
shufflevector <4 x float> %v, <4 x float> <float undef, float 1.0, float
undef, float undef>, <4 x i32> <i32 0, i32 5, i32 undef, i32 undef> ->
insertelement <4 x float> %v, float 1.0, 1

Differential Revision: https://reviews.llvm.org/D24182

llvm-svn: 282237

1 files changed, 49 insertions, 6 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index f3268d2c347..7d89a5f8e41 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -1037,17 +1037,21 @@ Value *InstCombiner::SimplifyDemandedVectorElts(Value *V, APInt DemandedElts,
       }
     }
 
-    APInt UndefElts4(LHSVWidth, 0);
+    APInt LHSUndefElts(LHSVWidth, 0);
     TmpV = SimplifyDemandedVectorElts(I->getOperand(0), LeftDemanded,
-                                      UndefElts4, Depth + 1);
+                                      LHSUndefElts, Depth + 1);
     if (TmpV) { I->setOperand(0, TmpV); MadeChange = true; }
 
-    APInt UndefElts3(LHSVWidth, 0);
+    APInt RHSUndefElts(LHSVWidth, 0);
     TmpV = SimplifyDemandedVectorElts(I->getOperand(1), RightDemanded,
-                                      UndefElts3, Depth + 1);
+                                      RHSUndefElts, Depth + 1);
     if (TmpV) { I->setOperand(1, TmpV); MadeChange = true; }
 
     bool NewUndefElts = false;
+    unsigned LHSIdx = -1u;
+    unsigned RHSIdx = -1u;
+    bool LHSUniform = true;
+    bool RHSUniform = true;
     for (unsigned i = 0; i < VWidth; i++) {
       unsigned MaskVal = Shuffle->getMaskValue(i);
       if (MaskVal == -1u) {
@@ -1056,18 +1060,57 @@ Value *InstCombiner::SimplifyDemandedVectorElts(Value *V, APInt DemandedElts,
         NewUndefElts = true;
         UndefElts.setBit(i);
       } else if (MaskVal < LHSVWidth) {
-        if (UndefElts4[MaskVal]) {
+        if (LHSUndefElts[MaskVal]) {
           NewUndefElts = true;
           UndefElts.setBit(i);
+        } else {
+          LHSIdx = LHSIdx == -1u ? MaskVal : LHSVWidth;
+          LHSUniform = LHSUniform && (MaskVal == i);
         }
       } else {
-        if (UndefElts3[MaskVal - LHSVWidth]) {
+        if (RHSUndefElts[MaskVal - LHSVWidth]) {
           NewUndefElts = true;
           UndefElts.setBit(i);
+        } else {
+          RHSIdx = RHSIdx == -1u ? MaskVal - LHSVWidth : LHSVWidth;
+          RHSUniform = RHSUniform && (MaskVal - LHSVWidth == i);
         }
       }
     }
 
+    // Try to transform shuffle with constant vector and single element from
+    // this constant vector to single insertelement instruction.
+    // shufflevector V, C, <v1, v2, .., ci, .., vm> ->
+    // insertelement V, C[ci], ci-n
+    if (LHSVWidth == Shuffle->getType()->getNumElements()) {
+      Value *Op = nullptr;
+      Constant *Value = nullptr;
+      unsigned Idx = -1u;
+
+      // Find constant vector wigth the single element in shuffle (LHS or RHS).
+      if (LHSIdx < LHSVWidth && RHSUniform) {
+        if (auto *CV = dyn_cast<ConstantVector>(Shuffle->getOperand(0))) {
+          Op = Shuffle->getOperand(1);
+          Value = CV->getOperand(LHSIdx);
+          Idx = LHSIdx;
+        }
+      }
+      if (RHSIdx < LHSVWidth && LHSUniform) {
+        if (auto *CV = dyn_cast<ConstantVector>(Shuffle->getOperand(1))) {
+          Op = Shuffle->getOperand(0);
+          Value = CV->getOperand(RHSIdx);
+          Idx = RHSIdx;
+        }
+      }
+      // Found constant vector with single element - convert to insertelement.
+      if (Op && Value) {
+        Instruction *New = InsertElementInst::Create(
+            Op, Value, ConstantInt::get(Type::getInt32Ty(I->getContext()), Idx),
+            Shuffle->getName());
+        InsertNewInstWith(New, *Shuffle);
+        return New;
+      }
+    }
     if (NewUndefElts) {
       // Add additional discovered undefs.
       SmallVector<Constant*, 16> Elts;